Hi!,
yep, all mine have munched on soil at some point or another.
I believe it is just a natural occurence and as long as there isnt anything that can cause impaction, its ok.
Everyday I watched Krunch go to the bottom of his enclosure, move a rock or 2, grab a little soil and come back up.
I will say I have noticed to has come to a halt since I started the Repashy All in One.
In my animal husbandry experience, both my horses and dogs have done this as well. I think it may be a missing mineral in their diet, but that is just my own observation.
So as long as you see healthy poop, keep the plant with the rocks on top in your cage. The other thing I did was add washed sand under the rocks, it helped with the gnats, not sure if they dont like the grit